Chemistry: Topic 6: The rate and extent of chemical change

The rate of a chemical reaction

How fast the reactants are changed into products

Examples of slow reactions

• one of the slowest is the rusting of iron

• chemical weathering - like acid rain damage to limestone buildings 

Examples of moderate speed reactions

• metal magnesium reacting with an acid to produce a gentle stream of bubbles

Examples of fast reactions

• burning

• explosions (faster and release lots of gas). They are over in a fraction of a second

Rate of reaction graphs

The steeper the line on the graph, the faster the rate of reaction. Over time, the line becomes less steep as the reactants are used up
The quickest reactions have the steepest lines and become flat in the least amount of time

What does the rate of chemical reaction depend on?

• Collision frequency of reacting particles

  • The more collision there are, the faster the reaction is

• Energy transferred during a collision

  • Particles have to collide with enough energy for the collision to be successful

  • Particles need the activation energy to break the bonds and start the reaction

• Factors that increase the number of collisions will increase the rate of reaction

Collision theory

Explains how various factors affect rates of reactions
According to this theory, chemical reactions can occur only when:

1. reacting particles collide with each other

2. with the right orientation

3. with sufficient energy

Activation energy

The minimum amount of energy that particles must have to react

Successful collision

A collision that ends in the particles reacting to form products

Factors affecting rate of reaction

1. Temperature

2. The concentration of a solution/ the pressure of a gas

3. Surface area

4. A catalyst

Catalyst

A substance that speeds up a reaction, without being used up in the reaction itself by providing an alternative reaction pathway with lower activation energy
It is not part of the overall reaction equation

Enzymes

Biological catalysts which speed up reactions in living things

Reaction profile for catalysts

Increasing the temperature increases the rate

• When a temperature is increased, the particles all move faster

• This means they will collide more frequently

• The faster they move, the more kinetic energy they have, so more of the collisions will have the minimum activation energy for the reaction to happen

Increasing the concentration or pressure increases the rate

• More concentrated solution means there are more particles the same volume of solvent

• When the pressure of a gas is increased, the same number of particles occupies a smaller space

• This makes collisions between the particles more frequent

Increasing the surface area increases the rate

• If one of the reactants is a solid, then breaking it up into smaller pieces will increase the surface area to volume ratio

• This means for the same volume, the particles around it have more area to collide on - so collisions will be more frequent 

Using a catalyst increases the rate

• Different catalysts are needed for different reactions, but they all work by decreasing the activation energy needed for the reaction to occur

• They do this by providing an alternative reaction pathway with lower activation energy

Mean rate of reaction (formula)

= quantity of reactant used / time taken
= quantity of product formed / time taken

Three ways to measure the rate of reaction

1. Precipitation and colour change
2. Change in mass (usually given off)
3. Volume of gas given off

Measuring the rate of reaction - precipitation and colour change

• You can record the visual change in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution (it becomes opaque)

• Observe a mark through the solution and measure how long it takes for it to disappear - the faster it disappears, the faster the reaction

• If the reactants are coloured and the products are colourless (or vice versa), you can time how long it takes for the solution to lose (or gain it takes) its colour

• The results are very subjective - different people may not agree over the exact point when the mark ‘disappears’ or the solution changes colour

• You can’t plot a graph from these results

Measuring the rate of reaction - change in mass (usually given off)

• Measuring the speed of a reaction that produces a gas can be carried out using a mass balance

• As the gas is released, the mass lost is measured on the balance

• The quicker the reading on the balance drops, the faster the reaction

• You can take measurements of the mass at regular intervals and you can plot a rate of reaction graph

• This is the most accurate of the three methods described because the mass balance is very accurate

• It has the disadvantage of releasing the gas straight into the room

Measuring the rate of reaction - volume of gas given off

• Uses a gas syringe to measure the volume of gas given off

• The more gas given off during a given time interval, the faster the reaction

• Gas syringes usually give volumes accurate to the nearest cm³, so are quite accurate

• You can take measurements at regular intervals and plot a rate of reaction graph using this method

• If the reaction is too vigorous, the plunger on the end of the syringe may blow

Calculate the mean reaction rate from a graph

= change in y value / change in x value (time taken usually)

This can be done for a whole graph or between two points in time
The rate finished when the graph goes flat

Calculate the reaction rate from a graph at a certain point

• Find the gradient of the curve (slope) at that point

• Draw a tangent - touches the curve at one point and doesn’t cross it, then find the gradient of the tangent

• Gradient of tangent = change in y / change in x

    = 𝚫y / 𝚫x

A reversible reaction

A reversible reaction occurs when the products of a reaction can react backwards to produce the original reactants

When is dynamic equilibrium reached?

In a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant

Factors affecting the position of equilibrium

• Temperature - for example: ammonium chloride > (heat) < (cool) ammonia + hydrogen chloride

• Pressure (only affects equilibria involving gases)

• Concentration of the reactants and products

Describe Le Chatelier’s Principle

If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract change and restore the equilibrium

Describe the effect of changing the concentration of reactant and product on the position of the equilibrium

• If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again

• If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again

• If the concentration of a product is decreased, more reactants will react until equilibrium is reached again

Describe the effect of changing temperature on the position of the equilibrium

If the temperature of a system at equilibrium is increased:

• the relative amount of products at equilibrium increases for an endothermic reaction

• the relative amount of products at equilibrium decreases for an exothermic reaction

Describe the effect of changing pressure on the position of the equilibrium

• This applies to equilibria that involve gases

• An increase in pressure favours the reaction with less molecules

• The equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction

• Pressure has no effect on the reactions where the numbers of gas molecules are equal on both sides of the equation

Describe the effect of a catalyst on the position of the equilibrium

• No effect

• It just speeds up both forward and backward reactions equally

• i.e. equilibrium is achieved faster

Le chatelier’s principle - Changes to temperature example

N₂ + 3H₂ ⇋ 2NH₃
The forward reaction is exothermic - a decrease in temp moves the equilibrium to the right

Le chatelier’s principle - Changes in pressure example

N₂ + 3H₂ ⇋ 2NH₃
There are 4 molecules on the left, but only 2 on the right
If you increase the pressure, the equilibrium shifts to the right

Le chatelier’s principle - Changes in concentration example

N₂ + 3H₂ ⇋ 2NH₃
If more reactants are added (LHS), the forward reaction increases to produce more NH₃